Fix TargetRegistry de/serialization and fix FixedPointIteratorPass constructor (#24867)

Fixed reproducer creation: The target registry is serialized as
`{target_registry=0}` in the reproducer file (for passes that contain
it, e.g. JitGlobalsPass). The deserialization when running a reproducer
thus fails, because it expects `{target_registry=global}`.


What changed?
Enabled -print-pipeline-passes
Fix TargetRegistry de/serialization
Fix FixedPointIteratorPass constructor

More detailed explanation of the fixes
- see
https://github.com/iree-org/llvm-project/blob/e65959f1684bd2416e8aa2ce684b8c300c381902/mlir/include/mlir/Pass/PassOptions.h#L77-L85
- Making the conversion from TargetRegistryRef to bool explicit, so that
`<<` is no longer overloaded for TargetRegistryDef (otherwise an
implicit conversion would be inserted, and simply 0 or 1 printed),
leading to the wrong output in the reproducer file
- Add a `parser<TargetRegistryRef>::print` overload that is selected
instead, printing either "global" or "unknown"
- Adapting TargetRegistryRef parsing; always select the global registry,
but print a warning if an argument different from "global" is provided

---------

Signed-off-by: Jakob Knauer <knauer@roofline.ai>
Co-authored-by: Tobias Fuchs <fuchs@roofline.ai>